1. Field of the Invention
This invention relates to a device to retain a dental prosthesis in the mouth, commonly known as a precision attachment. More particularly this invention concerns a system for securing a patient-detachable dental prosthesis to a retention bar which is mounted in the mouth, typically with the ends of the retention bar secured in implanted abutments or in crowns on teeth with the retention bar extending along and above the gum line where teeth are missing. A matching C-shaped component is included in the dental prosthesis which connects with the retention bar by means of a snap mechanism.
2. Discussion of Related Art
The retention clip and retention bar as described in the patent of Helmut Hader CH597843 are commercially available as PRECI-HORIX, see Brochure BRO-273.E-ed.B of Alphadent NV, Waregem, Belgium. This attachment system consists of a flexible C-shaped retention clip and a retention bar with a round section and a support section, an auxiliary spacer clip, special retention clips with reduced or stronger retention, a paralleling instrument for the retention bar, an insertion tool and a metal housing. These extra components facilitate the incorporation and the servicing of the precision attachment.
The paralleling instrument aligns the parallel walled support section of the retention bar to the path of insertion of the prosthesis.
The C-shaped retention clip has two flexible retention wings engaging the lateral sides of the round section of the retention bar. The retention clip is flexible and snaps by means of an insertion instrument behind retention ledges which have been provided in the dental prosthesis. The retention clip can easily be removed and replaced by a new retention clip with equal, less or more retention. The new retention clip is pressed in exactly the same place.
The ease of replacement of the retention clip is obtained by the use of a spacer clip during the processing of the dental prosthesis and which does not adhere to the acrylic resin of the dental prosthesis. After processing the acrylic resin of the dental prosthesis, the spacer clip is removed and will have created a void with a retention ledge which will retain the retention clip in place during function. The spacer clip also creates a passage for easier replacement of the retention clip. This passage is obtained by both extended spacer wings of the spacer clip which align along the support section of the retention bar.
Another property of the spacer clip is to align multiple retention clips parallel to the path of insertion of the dental prosthesis. As the extended spacer wings of the spacer clip all maintain close contact with the support section of the retention bar, parallelism and a neutral position are guaranteed for all retention clips.
In order to guarantee a perfect fit of the retention clip in the dental prosthesis, a metal housing is provided which has the exact inner shape of the retention clip and provides a retention ledge in metal rather than in the acrylic of the dental prosthesis which is easily damaged after a few replacements.
The dental prosthesis must be as stable as possible during its functional use. This presents special problems but is necessary to ensure comfort and functionality for the patient. In particular, when only two abutments are in the jaw which are connected with a retention bar, and the free-end saddles of the dental prosthesis also require support from the alveolar ridge, it is difficult to obtain a balance of stability and flexibility.
The mucosa which provides support to the free-end saddles can be slightly compressed and the alveolar ridge is bound to atrophy in time. This implies that the dental prosthesis must allow for a hinging movement in the direction of the compressed tissue or atrophied ridge. If the connection of the dental prosthesis to the retention bar is too rigid, the precision attachments and the abutments with supporting bone structure can be damaged.
The precision attachment described in the patent of Helmut Hader CH5957843 will allow for this movement. The dental prosthesis is able to rotate around the axis of the round section of the retention bar and allows the free-end saddles to compress the mucosa without putting torque on the retention bar and its abutments. This movement is commonly called resilience. The free-end saddles of the prosthesis, however, can also rotate in the opposite direction and lift off from the soft tissue when the patient is chewing sticky food or pressing with the tongue against the front teeth. This is particularly troublesome and causes discomfort and embarrassment to the patient.
Another frequent problem encountered with the above precision attachment is that the round section does not align with the retention wings of the retention clip when the patient places the dental prosthesis in his mouth. This causes the retention wings to be bent inwards and destroy the retention clip.
The patent application by Niklaus Gerhard DE3839112 describes a solution for a retention bar and a metal retention clip to allow rotation of the dental prosthesis in only one direction. The retention bar is asymmetrical in cross section being on one side U-shaped and on the other side ovoid shaped. This system has some drawbacks as it is not based on the use of a spacer clip which provides the advantages of ease of processing of the dental prosthesis and ease of replacement of the retention clips. The metal retention clip obtains snap retention at the ovoid side of the bar only, necessitating a long and flexible retention wing engaging the ovoid shape. A further drawback is that there is no provision to process the metal retention clip in a neutral position being in contact with the U-shaped surface. This is of considerable concern when the metal retention clip needs to be replaced in the patient's mouth with autopolymerizing resin.
The patent application by Milosevic Bozidar EP0170555 describes a metal retention clip which engages unilaterally with the undercut of an asymmetrical retention bar and simultaneously engages with parallel walled segments of the retention bar. This type of attachment does not have an indirect retainer in combination with a resilient dental prosthesis as this invention does.